The Ethernet Standard (IEEE 802.3) has recently been extended to include optical communications links operating at the speed of 10 Gb/s (IEEE Draft P802.3ae 2002). In this application, the IEEE 802.3 standard with its supplements will simply be referred to as the standard, and will be incorporated herein by reference.
For communication links in Local Area Network (LAN) and Metropolitan Area Network (MAN) environments, the new type of links (also known as 10GBASE) provides a lower cost alternative to SONET OC192 which has traditionally been employed for optical communications links to carry payload data of the order of 10 Gb/s between distant nodes.
Note that the Ethernet Standard IEEE 802.3 describes a number of variations of the 10GBASE family of links, combinations of suffixes such as R, X, S and L, M, and W referring to variants of format and of optical characteristics (see clause 44 of the standard). A variant of interest in Metropolitan environments is denoted 10GBASE-R (clause 49 of the standard). The 10GBASE standard also includes an optional format for embedding 10 Gb/s Ethernet signals at a slightly lower bit rate, as a payload within a SONET OC192 signal (10GBASE-W, clause 50 of the standard). The primary use of 10GBASE-W is in MANs and Wide Area Networks (WAN) where SONET is the predominant infrastructure.
While SONET provides for a variety of payload types which may include synchronous data as well as packet data, the 10GBASE-R links are designed to carry Ethernet packets directly, with a much simpler overhead structure than SONET.
SONET, having originally been developed as a universal optical transport protocol including long haul, provides a large number of overhead features, many of which may not be needed in a MAN environment, especially in cases where the only payload is packet data.
Ethernet on the other hand, having originally been developed as a LAN medium (and having evolved in speed to 10 Gb/s, and in scope to MAN), lacks some overhead features which may be desirable when used directly to interconnect nodes in a MAN environment. In many cases, with a packet transport medium, link related messages could be carried in additional packets, alongside the packets that carry user data.
This practice has a number of undesirable consequences: packet bandwidth that is used in carrying link related data is not available as user packet bandwidth; user packets and link related packets are distinguished only through their packet headers, increasing the possibility of malfunctions caused by incorrect headers, malicious packets, or decoding errors; and it may be difficult to provide access to the packet stream for the insertion of link related messages at link ends, for example requiring extra buffers and causing unwanted delay.
Another concern is the reduced ability of the Ethernet formats to provide link maintenance and link supervision features, especially when compared to such features in SONET. For example, SONET provides extensive bit-error monitoring of links using bit interleaved parity (BIP), and alarms for reporting error conditions from the far end.
A 10GBASE-R link according to the prior art is illustrated in FIG. 1. Shown is a bidirectional 10GBASE-R link system 100 interconnecting at the physical level two nodes, Node “A” (102) and Node “B” (104). The system comprises two 10GBASE-R transceivers 106 and 108 (commonly referred to as physical layer interface, or “PHY”, devices), interconnected by a transmission link 110. The transmission link 110 is also referred to as a “10 Gb/s Ethernet Link”. Electrical “10 Gigabit Attachment Unit Interfaces” (XAUI) 112 and 114 provide access to the link from other equipment (not shown) in the nodes 102 and 104 respectively.
Each of the transceivers 106 and 108 comprises a number of adaptation modules 116-122 to convert signals between the XAUI interfaces and the 10 Gb/s Ethernet Link 110, as well as a control module 124 for controlling the modules 116-122 and other devices. The transceivers 106 and 108 may also include electro-optical devices which are not shown.
In conformance with the standard, the conversion is done in two steps through intermediate internal interfaces 126 and 128 referred to as XGMII or “10 Gigabit Media Independent Interface”.
The adaptation modules are of two types, “XGMII Extender Sublayer” module types (XGXS) 116 and 118 for converting between XAUI (112, 114) and XGMII (126, 128) in each direction, and “Physical Coding Sublayer” module types (PCS) 120 and 122. For a 10 Gb/s Ethernet Link 110 according to the 10GBASE-R standard, the PCS modules 120 and 122 provide conversion in each direction between the XGMIIs (126, 128) and the specific signal format that is used on the 10 Gb/s Ethernet Links (110) based on the 10GBASE-R standard.
Also provided in the standard for 10GBASE-R are definitions for device control using a “Management Data Input/Output” interface (MDIO). The modules for device control (Control 124) shown in FIG. 1 are representative of means for controlling the interface devices, through their MDIO interfaces (not shown), and to communicate with other control structures (not shown) within the associated node.
The use of links based on the 10GBASE-R standard in environments where previously SONET OC-192 links might have been considered, leads to a significant cost reduction. Nevertheless, it is desirable to be able to transmit various types of link related information between the nodes interconnected by 10GBASE-R links without encroaching on the packet bandwidth that is committed to user data.
An example of a feature commonly employed on optical links is monitoring of the bit error rate (BER). The 10GBASE-R standard protocol provides only a rudimentary possibility for ongoing BER monitoring, for example by monitoring the correct appearance of the synchronization bits (sync bits, see FIGS. 44A-1 and 44A-2 of the standard). In the 10GBASE-R link system 100 of FIG. 1, such a feature would be controlled by the Control block 124 to monitor a BER detector in the receiving PCS device 122 of a transceiver 106 or 108, and report the result to the local control structure (not shown) within the associated node, and possibly to a network management system (not shown). Monitoring the correct appearance of the synchronization bits (2 out of every 66 bits) and extrapolating a BER from this provides only a crude estimate of the true BER, because no bit errors in the other 64 out of 66 bits are detected.
Another example of a common optical network feature is digital optical monitoring (DOM) which may exist in a link system of the type shown in FIG. 1. DOM is used for monitoring the quality of the received optical signal. This information is then commonly reported to the local control structure (not shown) within the associated node.
Overhead capabilities in optical links based on SONET include Bit Interleaved Parity (BIP for bit error monitoring), optical path identity, and other parameters embedded in the link overhead. While providing a standard (the MDIO) and other means (DOM) for the local monitoring of module and link performance, the 10GBASE-R standard, in the interest of keeping the cost of such links at a minimum, does not provide capabilities for transmitting link related information directly to the other end of the link.
The 10GBASE-R standard does provide a protocol element for signaling a local fault to the far end. This capability is intended to report a condition indicating an inability of the node to communicate, and is not suitable for link monitoring.
To improve the suitability of 10GBASE-R links in optical networks, and to provide additional link related functions it becomes necessary to develop a method and associated means to permit the insertion and extraction of additional information, compatibly with the 10GBASE-R standard protocol, without encroaching on the user data bandwidth, and without unduly increasing the cost of such links.